Francisella tularensis is an extremely virulent bacterium that can be transmitted naturally by blood sucking arthropods. During mammalian infection, F. tularensis infects numerous types of host cells, including erythrocytes. As erythrocytes do not undergo phagocytosis or endocytosis, it remains unknown how F. tularensis invades these cells. Furthermore, the consequence of inhabiting the intracellular space of red blood cells (RBCs) has not been determined. Here, we provide evidence indicating that residing within an erythrocyte enhances the ability of F. tularensis to colonize ticks following a blood meal. Erythrocyte residence protected F. tularensis from a low pH environment similar to that of gut cells of a feeding tick. Mechanistic studies revealed that the F. tularensis type VI secretion system (T6SS) was required for erythrocyte invasion as mutation of mglA (a transcriptional regulator of T6SS genes), dotU, or iglC (two genes encoding T6SS machinery) severely diminished bacterial entry into RBCs. Invasion was also inhibited upon treatment of erythrocytes with venom from the Blue-bellied black snake (Pseudechis guttatus), which aggregates spectrin in the cytoskeleton, but not inhibitors of actin polymerization and depolymerization. These data suggest that erythrocyte invasion by F. tularensis is dependent on spectrin utilization which is likely mediated by effectors delivered through the T6SS. Our results begin to elucidate the mechanism of a unique biological process facilitated by F. tularensis to invade erythrocytes, allowing for enhanced colonization of ticks.

No abstract

Francisella tularensis is a highly infectious bacterium that causes the disease, tularemia.Humans acquire tularemia through inhalation, oropharyngeal exposure, or tick bites. A unique feature of F. tularensis during infection is its ability to invade erythrocytes. Recent evidence indicated that residing within an erythrocyte enhances the ability of F. tularensis to colonize ticks. Further results indicated that residing within erythrocytes protects F. tularensis from the low pH environment associated with the gut cells of a feeding tick. Iron has previously been shown to be utilized by F. tularensis to protect against killing by reactive oxygen species. We hypothesized that F. tularensis was obtaining iron from erythrocytes. This iron was being utilized by F. tularensis for protection against the oxidative stress associated with a low pH environment, such as that found in the gut cells of a tick. We first sought to determine whetherF. tularensis was capable of acquiring iron from erythrocytes. Our results indicated that F. tularensis bacteria incubated with red blood cells contained significantly more iron than those incubated in media alone. This suggested that F. tularensis bacteria were capable of acquiring iron from these host cells. Investigations are ongoing that will assess whether additional iron is responsible for protecting the F. tularensis bacteria from the low pH environment of the tick gut cells.

Francisella tularensis is a highly infectious bacterium that causes the disease, tularemia. Humans acquire tularemia through inhalation, oropharyngeal exposure, or tick bites. A unique feature of F. tularensis during infection is its ability to invade erythrocytes. Evidence has indicated that residing within an erythrocyte enhances the ability of F. tularensis to colonize ticks. Previous evidence indicated that F. tularensis bacteria that had been incubated with erythrocytes showed increased survival in an acidic environment (pH = 3.6), a condition consistent with that found in the gut cells of a tick. We wanted to further examine whether residing within an erythrocyte enhanced the ability of F. tularensis to survive in this acidic environment. To do so, experiments are currently being conducted in which F. tularensis LVS and a mutant strain (ΔmglA) incapable of invading erythrocytes are incubated with red blood cells. These cells are subsequently subjected to a low pH environment. Results of these investigations are pending the ongoing investigations.